Gas chromatography/mass spectrometry (GC/MS) is an analytical method that combines the features of gas-chromatography and mass spectrometry to identify different substances within a test sample.
In some GC/MS instruments, ions are produced by electron ionization (EI) in an EI source, and then transferred to a downstream mass analyzer (e.g., a quadrupole mass filter) for examination. In some cases, direct current (DC) electrode lenses can be used to focus ions into a downstream vacuum stage and into the entrance of the mass analyzer to improve the collection of ions.